  We numerically investigate the relaxation dynamics in an isolated quantum
system of interacting bosons trapped in a double-well potential after an
integrability breaking quench. Using the statistics of the spectrum, we
identify the postquench Hamiltonian as nonchaotic and close to integrability
over a wide range of interaction parameters. We demonstrate that the system
exhibits thermalization in the context of the eigenstate thermalization
hypothesis (ETH). We also explore the possibility of an initial state to
delocalize with respect to the eigenstates of the postquench Hamiltonian even
for energies away from the middle of the spectrum. We observe distinct regimes
of equilibration process depending on the initial energy. For low energies, the
system rapidly relaxes in a single step to a thermal state. As the energy
increases towards the middle of the spectrum, the relaxation dynamics exhibits
prethermalization and the lifetime of the metastable states grows. Time
evolution of the occupation numbers and the von Neumann entropy in the
mode-partitioned system underpins the analyses of the relaxation dynamics.
