  We show that the conductance $G$ of molecular tunnel junctions wherein the
charge transport is dominated by a single energy level can be expressed in
closed analytic form which is exact and valid at arbitrary temperature $T$ and
model parameter values. On this basis, we show that the single-step tunneling
mechanism is compatible with a continuous thermal transition from a weakly
$T$-dependent $G$ at low $T$ (Sommerfeld regime) to a nearly exponential
$1/T$-dependent $G$ at high $T$ (Arrhenius-like regime). We predict that this
Sommerfeld-Arrhenius transition can be observed in real molecular junctions %
(e.g., based on perylene diimide) and can be continuously tuned, e.g., via
mechanical stretching.
