  We image the flow of a nearly random close packed, hard-sphere colloidal
suspension (a `paste') in a square capillary using confocal microscopy. The
flow consists of a `plug' in the center while shear occurs localized adjacent
to the channel walls, reminiscent of yield-stress fluid behavior. However, the
observed scaling of the velocity profiles with the flow rate strongly contrasts
yield-stress fluid predictions. Instead, the velocity profiles can be captured
by a theory of stress fluctuations originally developed for chute flow of dry
granular media. We verified this behavior both for smooth and rough boundary
conditions.
