  Warming in complex physical systems, in particular global warming, attracts
significant contemporary interest. It is essential, therefore, to understand
basic physical mechanisms leading to overheating. It is well known that
application of an electric field to conductors heats electric charge carriers.
Often an elevated electron temperature describes the result of the heating.
This paper demonstrates that an electric field applied to a conductor with
discrete electron spectrum produces a non-equilibrium electron distribution,
which cannot be described by temperature. Such electron distribution changes
dramatically the conductivity of highly mobile two dimensional electrons in a
magnetic field, forcing them into a state with a zero differential resistance.
Most importantly the results demonstrate that, in general, the effective
overheating in the systems with discrete spectrum is significantly stronger
than the one in systems with continuous and homogeneous distribution of the
energy levels at the same input power.
