  The method of microcontact spectroscopy in the superconducting state was used
to investigate weak nonlinearities of the current-voltage characteristics of
point contacts made of $N{{b}_{3}}Sn$ single crystals. The nature of the
spectrum of the electron-phonon interaction was found to vary considerably from
contact to contact, indicating considerable deviations of the composition of
the surface of $N{{b}_{3}}Sn$ from stoichiometry. A correlation was established
between the nature of the spectrum and the magnitude of the gap singularities
of the current-voltage characteristics. In the case of "dirty" high-resistance
contacts with strong gap singularities the microcontact spectra were reasonably
reproducible, which made it possible to relate them sufficiently closely to the
microcontact function of the electron-phonon interaction in the bulk material.
It was found that microcontact spectroscopy of this interaction was possible in
the superconducting state not only in dirty $S-c-S$ contacts, but also in dirty
$S-c-N$ contacts.
