  In this article, we study the thermalizability of a system consisting of two
atoms in a circular, transversely harmonic waveguide in the multimode regime.
While showing some signatures of the quantum-chaotic behavior, the system fails
to reach a thermal equilibrium in a relaxation from an initial state, even when
the interaction between the atoms is infinitely strong. We relate this
phenomenon to the previously addressed unattainability of a complete quantum
chaos in the Seba billiard [P. Seba, Phys. Rev. Lett., 64, 1855 (1990)], and we
conjecture the absence of a complete thermalization to be a generic property of
integrable quantum systems perturbed by a non-integrable but well localized
perturbation.
