simple exponential. Berne and Pecora in fact prove exactly the opposite, namely the contrapositive of their result is that if the relaxation is not a single exponential, then the Langevin model must not be applicable to the system, and therefore invocation of the the Langevin Model prediction eq \[eq:g1swrong\] is incorrect.

Berne and Pecora’s discussion refers purely and exclusively to the special case of particle motion that is described by the Langevin equation, in which case eqs \[eq:gaussianform\]-\[eq:g1ssimple\] are all correct. This special case corresponds to most of the experiments that were of interest at the time that ref was written, namely applications of quasielastic scattering for particle sizing[@dahneke1983]. For diffusing dilute colloids, the $t$ and $q$ dependences of $g^{(1s)}(q,t)$ are precisely as predicted by the Langevin model, in particular $g^{(1s)}(q,t) \sim  \exp(- \Gamma t)$ with $\Gamma \propto q^{2}$. The quasielastic scattering spectrum only leads to the time-dependent mean-square particle displacement if the spectrum is a pure exponential in $q^{2} t$. In this special case, the mean-square displacement increases linearly with $t$, so that the short-time and long-time behaviors of $g^{(1s)}(q,t)$ are one and the same.

If the decay of the field correlation function is not a simple exponential in 