  Utilizing a novel and exact inversion scheme, we determine the complex linear
conductivity $\sigma (\omega )$ from the linear magnetic ac-susceptibility
which has been measured from 3\,mHz to 50\,MHz in fields between 0.4\,T and
4\,T applied parallel to the c-axis of a 250\,nm thin disk. The frequency
derivative of the phase $\sigma ''/\sigma '$ and the dynamical scaling of
$\sigma (\omega)$ above and below $T_g(B)$ provide clear evidence for a
continuous phase transition at $T_g$ to a generic superconducting state. Based
on the vortex-glass scaling model, the resulting critical exponents $\nu $ and
$z$ are close to those frequently obtained on films by other means and
associated with an 'isotropic' vortex glass. The field effect on
$\sigma(\omega)$ can be related to the increase of the glass coherence length,
$\xi_g\sim B$.
