Using analytical arguments and computer simulations we show that the dependence of the hopping carrier mobility on the electric field $mu(F)/mu(0)$ in a system of random sites is determined by the localization length $a$, and not by the concentration of sites $N$. This result is in drastic contrast to what is usually assumed in the literature for a theoretical description of experimental data and for device modeling, where $N^{-1/3}$ is considered as the decisive length scale for $mu(F)$. We show that although the limiting value $mu(F rightarrow 0)$ is determined by the ratio $N^{-1/3}/a$, the dependence $mu(F)/mu(0)$ is sensitive to the magnitude of $a$, and not to $N^{-1/3}$. Furthermore, our numerical and analytical results prove that the effective temperature responsible for the combined effect of the electric field $F$ and the real temperature $T$ on the hopping transport via spatially random sites can contain the electric field only in the combination $eFa$.
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