  Based on the relaxation time approximation, the mean collision time for
magnon scattering $\tau_c(T)$ is computed from the experimental spin Seebeck
coefficient of a bulk YIG / Pt bilayer. The scattering results to be composed
by two processes: the low temperature one, with a $T^{-1/2}$ dependence, is
attributed to the scattering by defects and provides a mean free path around 10
$\mu$m; the high temperature one, depending on $T^{-4}$, is associated to the
scattering by other magnons. The results are employed to predict the thickness
dependence of the spin Seebeck coefficient for thin films.
