A theoretical approach is presented for predicting the collision‐based axial loss rate of a plasma confined by a Malmberg‐Penning trap, just after the axial well depth is made shallower. The assumptions used in developing the approach include a square well, uniform plasma properties, a particle mean‐free‐path that is much larger than the plasma dimensions, and a velocity distribution that is Maxwellian except within the regions of direct loss in velocity space. Example predictions are provided, and comparisons with existing theories are made. © 2003 American Institute of Physics