Momentum

Relaxation time= mean collision time/(1-)

where A is the average angle of scattering that the electron makes with its initial direction

average time, an electron spends between two successive collision, is called relaxation time and time spent by electron at point of contact, is called collision time

Rest and Relaxation or, Rest and Recreation.

Drift velocity refers to a particle's average velocity being influenced by its electric field. Momentum relaxation time is the time required for the inertial momentum of a particle to become negligible.