  It is revealed that in spin helix state of (001) quantum well system, strong
suppression of D'yakonov-Perel' spin relaxation process occurs by an interplay
between Rashba and Dresselhaus couplings over a wide range of Rashba coupling
strength. Contrary to common belief in early works, this leads to the finding
that Datta-Das type spin-field effect transistor is actually applicable to more
realistic non-ballistic transport regime in two dimensional electron gas
system.
