  Spin-orbit (SO) interactions in two dimensional systems split the Fermi
surface, and allow for the spatial separation of spin-states via transverse
magnetic focusing (TMF). In this work, we consider the case of combined Rashba
and Zeeman interactions, which leads to a Fermi surface without cylindrical
symmetry. While the classical trajectories are effectively unchanged, we
predict an additional contribution to the phase, linear in the applied in-plane
magnetic field. We show that this term is unique to TMF, and vanishes for
magnetic (Shubnikov de Haas) oscillations. Finally we propose some experimental
signatures of this phase.
