Mass transport is described by a mobility b, defined by: \[b_{i} = \frac{\nu _{i}}{\boldsymbol{F}_{i}}\] where ν i is the average drift velocity and Fi is the force on particle i.
Source:
PAC, 1999, 71, 1307. 'Definitions of Terms for Diffusion in the Solid State' on page 1319 (https://doi.org/10.1351/pac199971071307)