Modern technology makes it possible to deposit metal conductors with a width d of several nanometers onto a dielectric substrate using molecular epitaxy. Electrons move in such conductors practically without scattering and therefore they are actually waveguides for electron waves, and therefore, even at T = 0 K, the conductivity of the bridge at a sufficiently low electron concentration turns out to be zero. Starting from what values of the surface electron density n is the resistance of such a bridge

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