  We study models of interacting fermions in one dimension to investigate the
crossover from integrability to non-integrability, i.e., quantum chaos, as a
function of system size. Using exact diagonalization of finite-sized systems,
we study this crossover by obtaining the energy level statistics and Drude
weight associated with transport. Our results reinforce the idea that for
system size $L \to \infty$ non-integrability sets in for an arbitrarily small
integrability-breaking perturbation. The crossover value of the perturbation
scales as a power law $\sim L^{-3}$ when the integrable system is gapless and
the scaling appears to be robust to microscopic details and the precise form of
the perturbation. We conjecture that the exponent in the power law is
characteristic of the random matrix ensemble describing the non-integrable
system. For systems with a gap, the crossover scaling appears to be faster than
a power law.
