Context :

Aims : Ambipolar diffusion can cause a velocity drift between ions and neutrals .
This is one of the non-ideal magnetohydrodynamcis ( MHD ) effects proposed to enable the formation of large-scale Keplerian disks with sizes of tens of au .
To observationally study ambipolar diffusion in collapsing protostellar envelopes , we compare here gas kinematics traced by ionized and neutral molecular lines and discuss the implication on ambipolar diffusion .

Methods : We analyzed the data of the H ^ { 13 } CO ^ { + } ( 3–2 ) and C ^ { 18 } O ( 2–1 ) emission in the Class 0 protostar B335 obtained with our ALMA observations .
We constructed kinematical models to fit the velocity structures observed in the H ^ { 13 } CO ^ { + } and C ^ { 18 } O emission and to measure the infalling velocities of the ionized and neutral gas on a 100 au scale in B335 .

Results : A central compact ( \sim 1 \arcsec –2 \arcsec ) component that is elongated perpendicular to the outflow direction and exhibits a clear velocity gradient along the outflow direction is observed in both lines and most likely traces the infalling flattened envelope .
With our kinematical models , the infalling velocities in the H ^ { 13 } CO ^ { + } and C ^ { 18 } O emission are both measured to be 0.85 \pm 0.2 km s ^ { -1 } at a radius of 100 au , suggesting that the velocity drift between the ionized and neutral gas is at most 0.3 km s ^ { -1 } at a radius of 100 au in B335 .

Conclusions : The Hall parameter for H ^ { 13 } CO ^ { + } is estimated to be \gg 1 on a 100 au scale in B335 , so that H ^ { 13 } CO ^ { + } is expected to be attached to the magnetic field .
Our non-detection or upper limit of the velocity drift between the ionized and neutral gas could suggest that the magnetic field remains rather well coupled to the bulk neutral material on a 100 au scale in this source , and that any significant field-matter decoupling , if present , likely occurs only on a smaller scale , leading to an accumulation of magnetic flux and thus efficient magnetic braking in the inner envelope .
This result is consistent with the expectation from the MHD simulations with a typical ambipolar diffusivity and those without ambipolar diffusion .
On the other hand , the high ambipolar drift velocity of 0.5–1.0 km s ^ { -1 } on a 100 au scale predicted in the MHD simulations with an enhanced ambipolar diffusivity by removing small dust grains , where the minimum grain size is 0.1 \mu m , is not detected in our observations .
However , because of our limited angular resolution , we can not rule out a significant ambipolar drift only in the midplane of the infalling envelope .
Future observations with higher angular resolutions ( \sim 0 \aas@@fstack { \prime \prime } 1 ) are needed to examine this possibility and ambipolar diffusion on a smaller scale .