Relaxation-Time Approximation for the Mobility of Electrons in Helium

Abstract
Kubo's theory of irreversible quantum statistics is applied to the problem of electron mobility in a hardcore gas. Without any assumptions regarding a relaxation time, the mobility is evaluated by cumulant techniques to first order in the gas density and second order in the scattering length. A momentum-dependent relaxation time appears in a natural manner, as does the criterion for validity of the classical Langevin theory.