  The mobility $\mu$ of excess electrons in dense Argon gas has been measured
as a function of the applied electric field $E$ and of the gas density N at
several temperatures in the range $142.6<T<200$ K, encompassing the critical
temperature Tc=150.86 K. We report here measurements at densities up to N?7
nm$^{-3}$, close to the critical density, $N_c \approx$ 8.1 nm$^{-3}$, reached
for the isotherm closest to the critical one. At all temperatures, below as
well as above Tc, and up to moderately high densities, the density-normalized
mobility $\mu N$ shows the usual electric field dependence present in a gas
with a Ramsauer-Townsend minimum due to the mainly attractive electron-atom
interaction. mN is constant and field independent for small values of $E,$
shows a maximum for a reduced field $E/N \approx$ 4 mTd, and then decreases
rapidly with the field, approximately proportional to $(E/N)^{-1/2}.$ The
zero-field density-normalized mobility $\mu_0 N, $for all T>$T_c,$ shows the
well known anomalous positive density effect, i.e., m0N increases with
increasing N, confirming previous results obtained for T=300 K and 162.7 K.
Below Tc, however, $\mu_0 N$ does not show the expected anomalous positive
density effect, but it rather features a broad maximum. This appears to be a
crossover behavior between the positive density effect shown for T>Tc and the
small negative effect previously observed for T$\approx$ 90 K. In any case, the
data at all temperatures confirm the interpretation of the anomalous density
effect as being essentially due by the density-dependent quantum shift of the
electron ground state kinetic energy in a disordered medium as a result of
multiple scattering (MS) processes, although other MS processes do influence
the outcome of the experiment.
