  In strong-coupling superconductors with a short electron mean free path the
self-energy effects in the superconducting order parameter play a major role in
the phonon manifestation of the point-contact spectra at above-gap energies. We
compare the expressions for the nonlinear conductivity of tunnel, ballistic,
and diffusive point-contacts and show that these expression are similar and
correspond to the measurements of the phonon structure in the point-contact
spectra for the $\pi$-band of MgB$_{2}$.
