  The mobility of surface electrons localized over helium films underlying
solid substrates has been evaluated by solving the Boltzmann equation in the
time relaxation approximation and the force balance equation in which an
effective mobility is obtained in terms of the dynamical structure factor of
the nondegenerate electron liquid. The essential processes of electron
scattering by gas atoms, ripplons, and film-solid interface roughness are taken
into account. The electron mobility dependence on the film thickness and
temperature is determined and compared with experimental data available. We
find that the interface-roughness scattering is the dominant process for
explaining the experimental results. We estimate the extended defect sizes of
the underlying substrate within the Gaussian correlated model for interface
roughness.
