  A phenomenological theory of mode collapse-revival for a system with a weak
nonlinearity is presented. The theory takes into account fluctuations of the
number n of quanta of oscillations. The collapse time $\tau _c ^{-1}$ for a
mode of frequency $\omega _0$ turns out to be $\omega _0(\sqrt{n/2}\hbar \omega
_0 \mid \nu \mid)$, where \nu is a coefficient in a nonlinear correction to the
mode frequency with respect to the oscillation energy E. For a Bose gas in a
harmonic trap $\tau _c$ is order of $\omega _0^{-1}\sqrt{\mu N/E\hbar \omega
_0}$. This value is order of 250 ms. for typical experimental conditions. The
coefficient \nu is calculated for the breathing mode in an isotropic trap.
