  A curved two dimensional electron gas with spin-orbit interactions due to the
radial confinement asymmetry is considered. At certain relation between the
spin-orbit coupling strength and curvature radius the tangential component of
the electron spin becomes a conserved quantity for any spin-independent
scattering potential that leads to a number of interesting effects such as
persistent spin helix and strong anisotropy of spin relaxation times. The
effect proposed can be utilized in the non-ballistic spin-field-effect
transistors.
