  We use a mean field approach to study the conductance modulation of gate
controlled semiconductor spin interferometers based on the Rashba spin-orbit
coupling effect. The conductance modulation is found to be mostly due to
Ramsauer type transmission resonances rather than the Rashba effect in typical
structures. This is because of significant reflections at the interferometer's
contacts due to large potential barriers and effective mass mismatch between
the contact material and the semiconductor. Thus, unless particular care is
taken to eliminate these reflections, any observed conductance modulation of
spin interferometers may have its origin in the Ramsauer resonances (which is
unrelated to spin) rather than the Rashba effect.
