  The dependence of spin relaxation on the direction of the quantum wire under
Rashba and Dresselhaus (linear and cubic) spin orbit coupling is studied.
Comprising the dimensional reduction of the wire in the diffusive regime, the
lowest spin relaxation and dephasing rates for (001) and (110) systems are
found. The analysis of spin relaxation reduction is then extended to
non-diffusive wires where it is shown that, in contrast to the theory of
dimensional crossover from weak localization to weak antilocalization in
diffusive wires, the relaxation due to cubic Dresselhaus spin orbit coupling is
reduced and the linear part shifted with the number of transverse channels.
