  An analytical expression is derived for the conductance modulation of a
ballistic two dimensional Datta-Das Spin Field Effect Transistor (SPINFET) as a
function of gate voltage. Using this expression, we show that the recently
observed conductance modulation in a two-dimensional SPINFET structure does not
match the theoretically expected result very well. This calls into question the
claimed demonstration of the SPINFET and underscores the need for further
careful investigation.
