mean free path 

The vacuum distillation section says that Li-6 has a greater mean free path (than Li-7). However, the mean free path (of an ideal gas) does not depend on the speed or mass of the particles. 

,

where k is the Boltzmann constant, T is temperature, σ is collision cross-section, p is pressure.

Since Li-6 is lighter, it most probably evaporates faster than Li-7 (since equal kinetic energy results in larger speed for Li-6, more Li-6 atoms are fast enough to escape), but it has nothing to do with mean free path. Szaszicska, from huwiki