  Diagram, known in theory of the Anderson localization as the Hikami box, is
computed for the Sinai billiard. This interference effect is mostly important
for trajectories tangent to the opening of the billiard. This diagram is
universal at low frequencies, because of the particle number conservation law.
An independent parameter, which we call phase volume of diffraction, determines
the corresponding frequency range. This result suggests that level statistics
of a generic chaotic system is not universal.
