  We report effective mass (m*) measurements, via analyzing the temperature
dependence of the Shubnikov-de Haas oscillations, for dilute, interacting,
two-dimensional electron systems (2DESs) occupying a single conduction-band
valley in AlAs quantum wells. When the 2DES is partially spin-polarized, m* is
larger than its band value, consistent with previous results on various 2DESs.
However, as we fully spin polarize the 2DES by subjecting it to strong parallel
magnetic fields, m* is unexpectedly suppressed and falls even below the band
mass.
