  We develop a general kinetic theory framework to describe the hydrodynamics
of strongly interacting, nonequilibrium quantum systems in which integrability
is weakly broken, leaving a few residual conserved quantities. This framework
is based on a generalized relaxation-time approximation; it gives a simple, but
surprisingly accurate, prescription for computing nonequilibrium transport even
in strongly interacting systems. This approximation reproduces the crossover
from generalized to conventional hydrodynamics in interacting one-dimensional
Bose gases with integrability-breaking perturbations, both with and without
momentum conservation. It also predicts the hydrodynamics of chaotic quantum
spin chains, in good agreement with matrix product operator calculations.

Statistical Mechanics;Quantum Gases;Strongly Correlated Electrons;Quantum Physics