  Transport of spherical Brownian particles of finite size possessing radii
through narrow channels with varying cross-section area is considered. Applying
the so-called Fick-Jacobs approximation, i.e. assuming fast equilibration in
orthogonal direction of the channel axis, the 2D problem can be described by a
1D effective dynamics in which bottlenecks cause entropic barriers. Geometrical
confinements result in entropic barriers which the particles have to overcome
in order to proceed in transport direction. The analytic findings for the
nonlinear mobility for the transport are compared with precise numerical
simulation results. The dependence of the nonlinear mobility on the particle
size exhibits a striking resonance-like behavior as a function of the relative
particle size; this latter feature renders possible new effective particle
separation scenarios.
