  The density matrix formalism is applied to calculate the spin-relaxation time
for two-dimensional systems with a hierarchy of spin-orbit couplings, such as
Rashba-type, Dresselhaus-type and so on. It is found that the spin-relaxation
time can be infinite if those coupling strengths $\alpha$, $\beta$, $\gamma_1$
and $\gamma_2$ satisfy either condition (i) $\alpha=\beta, \gamma_1=0$ or (ii)
$\alpha=-\beta, \gamma_2=0$, which correspond to the vanishing Yang-Mills
"magnetic" field. The effect caused by the application of an external magnetic
field is also discussed. It is found that the longitudinal and in-plane spin
components can possess infinite life time when the spin components, the Larmor
precession frequency and the external magnetic field satisfy certain relations.
