  Superconducting fluctuations in long and narrow strips made from ultrathin
NbN films, have been investigated. For large bias currents close to the
critical current fluctuations led to localized, temporary transitions into the
normal conducting state, which were detected as voltage transients developing
between the strip ends. We present models based on fluctuations in the Cooper
pair density and current-assisted thermal-unbinding of vortex-antivortex pairs,
which explain the current and temperature dependence of the experimental
fluctuation rates.
