  We present the results of the planar diffusion of a Dirac particle by step
and barrier potentials, when the incoming wave impinges at an arbitrary angle
with the potential. Except for right-angle incidence this process is
characterized by the appearance of spin flip terms. For the step potential,
spin flip occurs for both transmitted and reflected waves. However, we find no
spin flip in the transmitted barrier result. This is surprising because the
barrier result may be derived directly from a two-step calculation. We
demonstrate that the spin flip cancellation indeed occurs for each particle
(wave packet) contribution.
