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_c) surprisingly exhibits maxima for mismatching (defective) structures, originating from traffic-jam-like flow due to disorder in the edges. We then focus on the behavior for higher fields, approaching the 2D melting field B_m. 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 B_m, 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.
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