  An experimental investigation was made of the temperature dependence of the
first step of a phase-slip line in a thin superconducting tin film. The depth
of penetration of a nonequilibrium longitudinal electric field into the
superconductor was determined near the critical temperature. A comparison was
made with theoretical investigations of one-dimensional structures containing
phase-slip centers. The experimental results were found to be in good agreement
with the theory when the mechanism of mixing of electron-like and hole-like
branches of the quasiparticle spectrum was governed by the elastic scattering
of the excitations. This was one more experimental confirmation that a
phase-slip line is a two-dimensional analog of a phase-slip center.
