  We report measurements of excess electron mobility in dense Argon gas at the
two temperatures $T=152.15$ and 162.30 K, fairly close to the critical one
($T_c =150.7$ K), as a function of the gas density $N$ up to 14
atoms$\cdot$nm$^{-3}$ ($N_{c}=8.08$ atoms$\cdot$nm$^{-3}$). For the first time
a maximum of the zero-field density-normalized mobility $\mu_{0}N$ has been
observed at the same density where it was detected in liquid Argon under
saturated vapor pressure conditions. The existence of the $\mu_{0}N$ maximum in
the liquid is commonly attributed to electrons scattering off long-wavelength
collective modes of the fluid, while for the low-density gas a density-modified
kinetic model is valid. The presence of the $\mu_{0}N$ maximum also in the gas
phase raises therefore the question whether the single scattering picture valid
in the gas is valid even at liquid densities.
