  We measured and compared the electric field vs current density
characteristics in the vortex state of two amorphous Nb0.7Ge0.3 microbridges,
with and without a line of submicron holes patterned along the sample axis. The
power dissipation in the perforated sample exhibits a crossover, being reduced
at temperatures well below the superconducting transition temperature Tc and
unexpectedly enhanced close to Tc. At low temperatures the holes are efficient
artificial pinning centres and reduce the average vortex velocity. We argue
that the dissipation enhancement close to Tc is a consequence of a combination
of the weakened pinning by the holes and an inhomogeneous driving-current
distribution in their vicinity, which results in an increased average vortex
velocity as well as in a channeling of the vortex motion through the holes.
