Parallel nucleation field in thin superconducting films

Nucleation of superconductivity in a thin film situated in a parallel magnetic field is considered. The field dependence of the coherence length is taken into account. In particular, the known result for the nucleation field $H$ near the critical temperature $T_c$ is shown to hold only in the dirty limit. For a finite mean free path $H^2$ depends linearly on $T$ as in the dirty limit. However, the slope $\frac{d{H}^2}{\mathrm{dT}}$ at $T_c$ may change sign to positive for thin and clean enough films.