  Magneto-resistivity of a c-axis oriented MgB$_{2}$ thin film was studied in
perpendicular and parallel magnetic fields up to $\sim $23 T with temperatures
down to 0.38 K. Resistive critical magnetic fields were determined. Large
separations between irreversibility lines and upper critical magnetic fields
are observed. An effective quantum parameter as a function of temperature is
proposed to explain the separations under the theoretical framework of quantum
fluctuations, in good agreement with our experimental and previously published
experimental data.
