  Recent experiments have demonstrated long spin lifetimes in uniformly n-doped
quantum wells. The spin dynamics of exciton, localized, and conduction spins
are important for understanding these systems. We explain experimental behavior
by invoking spin exchange between all spin species. By doing so we explain
quantitatively and qualitatively the striking and unusual temperature
dependence in (110)-GaAs quantum wells. We discuss possible future experiments
to resolve the pertinent localized spin relaxation mechanisms. In addition, our
analysis allows us to propose possible experimental scenarios that will
optimize spin relaxation times in GaAs and CdTe quantum wells.
