The calculated τJ values do not appear to be related to the mean times between collisions calculated using a hard sphere model.

With increasing height, the atmosphere becomes more rarefied, which increases the mean time between collisions of molecules (also known as Mean free time).

This formula is valid away from strong atomic or molecular resonances (where would have a large imaginary part) and at frequencies which are much below both the material's plasma frequency (dependent on the density of free electrons in the material) and the reciprocal of the mean time between collisions involving the conduction electrons.