  The transport behavior of a system of gravitationally driven colloidal
particles is investigated. The particle interactions are determined by the
superparamagnetic behavior of the particles. They can thus be arranged in a
crystalline order by application of an external magnetic field. Therefore the
motion of the particles through a narrow channel occurs in well-defined lanes.
The arrangement of the particles is perturbed by diffusion and the motion
induced by gravity. Due to these combined influences a density gradient forms
along the direction of motion of the particles. A reconfiguration of the
crystal is observed leading to a reduction of the number of lanes. In the
course of the lane reduction transition a local melting of the
quasi-crystalline phase to a disordered phase and a subsequent crystallization
along the motion of the particles is observed. This transition is characterized
experimentally and using Brownian dynamics (BD) simulations.
