The Thouless energy is a characteristic energy scale of diffusive disordered conductors. It is defined by , where D is the diffusion constant and L the size of the system, and thereby inversely proportional to the diffusion time through the system.

It was first introduced by the Scottish-American physicist David J. Thouless when studying Anderson localization

Anderson localization

In condensed matter physics, Anderson localization, also known as strong localization, is the absence of diffusion of waves in a disordered medium. This phenomenon is named after the American physicist P. W...

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as a measure of the sensitivity of energy levels to a change in the boundary conditions of the system. Though being a classical quantity, it has been shown to play an important role in the quantum-mechanical treatment of disordered systems.