  The diffusion of energy that is locally deposited into two-dimensional
electron gases by Joule heating generates transverse voltages across devices
with broken symmetry. For mesoscopic structures characterized by device
dimensions comparable to the energy diffusion length, the resulting thermopower
strongly depends on details of the potential profile defined by electric gates.
We discuss these mesoscopic features within a diffusion thermopower model and
propose schemes to measure the energy diffusion length and its dependence on
gate voltage.
