  We present the results of a spectroscopic study of state-of-the-art MgB2
single crystals, carried out by using a modified point-contact technique. The
use of single crystals allowed us to obtain point contacts with current
injection either parallel or perpendicular to the ab planes. The effect of
magnetic fields up to 9 T on the conductance spectra of these contacts is here
thoroughly studied, for both B parallel and perpendicular to the ab planes. The
complete thermal evolution of the upper critical field of the "pi" band is
determined for the first time, and quantitative information about the upper
critical field of the "sigma" band and its anisotropy is obtained. Finally, by
exploiting the different effect of a magnetic field applied parallel to the ab
planes on the two band systems, the partial contributions of the "sigma" and
"pi" bands to the total conductance are obtained separately. Fitting each of
them with the standard BTK model yields a great reduction of the uncertainty on
Delta_sigma and Delta_pi, whose complete temperature dependence is obtained
with the greatest accuracy.
