We investigated the dynamics of vortex matter confined to mesoscopic channels by means of mode
locking experiments. When vortices move coherently through the pinning (shear) potential provided by static
vortices in the channel edges, interference between the washboard frequency of the lattice and the frequency of
superimposed rf-currents causes (Shapiro-like) steps in the dc-IV curves. These steps allow to trace directly how the
number of moving rows in each channel and the frustration between row spacing and channel width, varies with
magnetic field. The flow stress (~ I,) surprisingly exhibits maxima for mismatching (defective) structures, originating
from traftic-jam-like flow due to disorder in the edges. We then focus on the behavior for higher fields, approaching
the 2D melting field Bm. In this regime the presence of the interference phenomenon, characteristic for crystalline
motion, strongly depends on the velocity (applied frequency) at which vortices are probed. The minimum velocity to
observe coherent, solid-like motion is found to diverge when the field is increased towards $E_{''}$ above which the
interference is absent for any frequency. This provides the first direct evidence for a velocity dependent, dynamic
phase transition of vortex matter moving through disorder, as predicted by Koshelev and Vinokur.