  The magnetic field dependence of electron effective mass is an important
subject in consideration of physical properties of superconducting ferromagnets
URhGe, UCoGe and paramagnet UTe$_2$. It is usually accepted to find this
dependence from the field dependence of $A$ coefficient in the low temperature
resistivity law $\rho=\rho_0+AT^2$. It is shown here that the field dependence
$A(H)$ takes place also for field-independent effective mass. Whether or not
the effective mass depends on the magnetic field can be checked by the
measurements of the amplitude of de Haas - van Alphen magnetisation
oscillations.
