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In superconducting filaments of cross sections smaller than about 10^-14 m², in a large frequency range at all temperatures, collective density oscillations exist with a soundlike dispersion relation and weak damping. The restoring force is the Coulomb interaction. In the restricted geometry this force is proportional to the wavelength and hence does not shift the density mode to the bulk plasma frequency.
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Mooij et al. (1985) studied this question.
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