  Upper critical fields of four MgB2 thin films were measured up to 28 Tesla at
Grenoble High Magnetic Field Laboratory. The films were grown by Pulsed Laser
Deposition and showed critical temperatures ranging between 29.5 and 38.8 K and
resistivities at 40 K varying from 5 to 50 mWcm. The critical fields in the
perpendicular direction turned out to be in the 13-24 T range while they were
estimated to be in 42-57 T the range in ab-planes. In contrast to the
prediction of the BCS theory, we did not observe any saturation at low
temperatures: a linear temperature dependence is exhibited even at lowest
temperatures at which we made the measurements. Moreover, the critical field
values seemed not to depend on the normal state resistivity value. In this
paper, we analyze these data considering the multiband nature of
superconductivity in MgB2 We will show how the scattering mechanisms that
determine critical fields and resistivity can be different.
