  The temperature dependence of upper critical field B_c2 was determined from
the shift of resistive transition \Delta T(B) in nearly optimally doped
Nd_{2-x}Ce_xCuO_{4-y} single crystals. Within the experimental accuracy, the
weak-field data are described by power function B_c2 ~ (\Delta T)^{3/2}. This
result is compared with the data on heat capacity and analyzed in the context
of possible manifestations of boson effects in superconductivity. The
temperature dependence of B_c2 persists down to the lowest temperatures, but
the numerical values of B_c2 below 1 K are different for different samples.
