  We consider the impact of inertia on biased Brownian motion of point
particles in a two-dimensional channel with sinusoidally varying width. If the
time scales of the problem separate, the adiabatic elimination of the
transverse degrees of freedom leads to an effective description for the motion
along the channel given by the potential of mean force. The possibility of such
description is intimately connected with equipartition. Numerical simulations
show that in the presence of external bias the equipartition may break down
leading to non-monotonic dependence of mobility on external force and several
other interesting effects.
