  The nonlinearities of current-voltage characteristics (IVC) of
superconducting niobium point contacts for voltages of the order of the
characteristic phonon frequencies are investigated. It is shown that in
limiting by dirty contacts ${{l}_{1}}\ll d$ , (${{l}_{1}}$ is the momentum
mean-free path of electrons), features are observed on the IVC corresponding to
maxima in the first or second derivatives and are situated at energies close to
the characteristic phonon energies. These features are due to the energy
dependence of the excess current, since the nonlinearity of the IVC in the
normal state is several times smaller in absolute magnitude, and its second
derivative is monotonic. For a contact diameter $d$ of the order of several
tens of angstroms (the contact resistance is of the order of 100 $\Omega $),
the thermal effects can be neglected. It is assumed that in the region of the
constriction, the characteristic lengths of the system satisfy the relation
$\xi \gtrsim d>{{\Lambda }_{\varepsilon }}$ (${{\Lambda }_{\varepsilon }}$ is
the diffusion length for energy relaxation of the electrons, $\xi$ is the
coherence length) which makes possible inelastic transitions of electrons
between pair chemical potentials differing in energy by an account $eV$ ($V$ is
the voltage drop across the contact)
