  We report magnetoresistance measurements through Nb films having a periodic
thickness modulation. The cylinder shaped large-thickness regions of the
sample, which form a square lattice, act as repulsive centers for the
superconducting vortices. For low driving currents along one of the axes of the
square lattice, the resistivity $\rho$ increases monotonously with increasing
magnetic field $B$ and the $\rho$--$B$ characteristics are approximately
piecewise linear. The linear $\rho$ vs $B$ segments change their slope at
matching fields where the number of vortices is an integer or a half integer
times the number of protruding cylinders in the sample. Numerical simulations
allow us to associate the different segments of linear magnetoresistance to
different vortex-flow regimes, some of which are dominated by the propagation
of discommensurations (kinks).
