  We further analyze the recently proposed criterion, according to which a
superfluid transition occurs whenever the r.m.s. displacement of a particle
from its initial position in imaginary time reaches a certain fraction of the
interparticle distance. The critical temperature $T_\lambda$ is expressed in
terms of the single particle mobility. Within the Drude approximation for the
mobility $T_\lambda$ is determined by the friction coefficient (or by the
viscosity for dense liquids). To check the validity of the criterion the
resulting formula is applied to non-ideal Bose gas, to liquid helium and to
$^3$He-$^4$He mixtures. A reasonable qualitative agreement with available
experiments is obtained in all cases. In the case of the Bose gas the present
approach results also in an upper bound on the peak value of
$T_{\lambda}/T_{\lambda}^0$, where $T_{\lambda}^0$ is the critical temperature
for the ideal gas.
