  An effective spin relaxation mechanism that leads to electron spin
decoherence in a quantum dot is proposed. In contrast to the common
calculations of spin-flip transitions between the Kramers doublets, we take
into account a process of phonon-mediated fluctuation in the electron spin
precession and subsequent spin phase diffusion. Specifically, we consider
modulations in the longitudinal g-factor and hyperfine interaction induced by
the phonon-assisted transitions between the lowest electronic states. Prominent
differences in the temperature and magnetic field dependence between the
proposed mechanisms and the spin-flip transitions are expected to facilitate
its experimental verification. Numerical estimation demonstrates highly
efficient spin relaxation in typical semiconductor quantum dots.
