  Electron spin dynamics in InAs/GaAs heterostructures consisting of a single
layer of InAs (1/3$\sim$1 monolayer) embeded in (001) and (311)A GaAs matrix
was studied by means of time-resolved Kerr rotation spectroscopy. The spin
relaxation time of the sub-monolayer InAs samples is significantly enhanced,
compared with that of the monolayer InAs sample. We attributed the slowing of
the spin relaxation to dimensionally constrained
D\textquoteright{}yakonov-Perel\textquoteright{} mechanism in the motional
narrowing regime. The electron spin relaxation time and the effective g-factor
in sub-monolayer samples were found to be strongly dependent on the
photon-generated carrier density. The contribution from both
D\textquoteright{}yakonov-Perel\textquoteright{} mechanism and Bir-Aronov-Pikus
mechanism were discussed to interpret the temperature dependence of spin
decoherence at various carrier densities.
