Measurements of electron back-diffusion to plane cold cathodes in a uniform electric field in Ar do not agree satisfactorily with theories derived from first principles. The process has been re-analysed in terms of one-dimensional diffusion and drift with allowance made for the Ramsauer-Townsend effect, which causes the electron's energy and mean free path to vary from its initial value at the cathode to its equilibrium value in the applied field. The analytically obtained expression for the ratio of collected to emitted current densities j/j₀, which follows similarity rules, depends on pl, the product of gas pressure and electrode separation, and on the reduced field, E/p. It is found that for values of (E/p)³4/(pl)>or approximately=10, j/j₀ becomes independent of pl.