  We study the spin-relaxation time in materials where a large spin-orbit
coupling (SOC) is present which breaks the spatial inversion symmetry. Such a
spin-orbit coupling is realized in zincblende structures and heterostructures
with a transversal electric field and the spin relaxation is usually described
by the so-called D'yakonov-Perel' (DP) mechanism. We combine a Monte Carlo
method and diagrammatic calculation based approaches in our study; the former
tracks the time evolution of electron spins in a quasiparticle dynamics
simulation in the presence of the built-in spin-orbit magnetic fields and the
latter builds on the spin-diffusion propagator by Burkov and Balents [Burkov
and Balents Phys. Rev. B. 69, 245312 (2004).]. Remarkably, we find a parameter
free quantitative agreement between the two approaches and it also returns the
conventional result of the DP mechanism in the appropriate limit. We discuss
the full phase space of spin relaxation as a function of SOC strength, its
distribution, and the magnitude of the momentum relaxation rate. This allows us
to identify two novel spin-relaxation regimes; where spin relaxation is
strongly non-exponential and the spin relaxation equals the momentum
relaxation. A compelling analogy between the spin-relaxation theory and the NMR
motional narrowing is highlighted.
