  Numerically, we study the time fluctuations of few-body observables after
relaxation in isolated dynamical quantum systems of interacting particles. Our
results suggest that they decay exponentially with system size in both regimes,
integrable and chaotic. The integrable systems considered are solvable with the
Bethe ansatz and have a highly nondegenerate spectrum. This is in contrast with
integrable Hamiltonians mappable to noninteracting ones. We show that the
coefficient of the exponential decay depends on the level of delocalization of
the initial state with respect to the energy shell.
