  We report on time-resolved Kerr rotation measurements of spin coherence of
electrons in the first excited subband of a high-mobility low-density
two-dimensional electron system in a GaAs/Al0.35Ga0.65As heterostructure. While
the transverse spin lifetime (T2*) of electrons decreases monotonically with
increasing magnetic field, it has a non-monotonic dependence on the
temperature, with a peak value of 596 ps at 36 K, indicating the effect of
inter-subband electron-electron scattering on the electron spin relaxation. The
spin lifetime may be long enough for potential device application with
electrons in excited subbands.
