  We consider the mobility of electrons in an environment of static hard-sphere
scatterers, which provides a realistic description of electrons in Helium gas.
A systematic expansion in the scatterer density is carried to second order
relative to the Boltzmann result, and the analytic contribution at this order
is derived, together with the known logarithmic term in the density expansion.
It is shown that existing experimental data are consistent with the existence
of the logarithmic term in the density expansion, but more precise experiments
are needed in order to unambiguously detect it. We show that our calculations
provide the necessary theoretical information for such an experiment, and give
a detailed discussion of a suitable parameter range.
