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The orbital magnetic moment and the orbital magnetic susceptibility at finite temperature for a charged oscillator in a magnetic field of arbitrary strength are calculated exactly. Landau diamagnetism, a purely quantum mechanical non-vanishing diamagnetic susceptibility in the absence of a magnetic field and the Langevin formula are obtained as special cases.

The electrical resistivity p(d, T ) of single-crystalline copper whiskers with diameters d between 7 and 8 1 p n and residual electron mean free path I, = 240 pm has been measured in the temperature range 0.4 K g Tg4.2 K. The magnitude of the temperaturedependent part Ap(d, T ) turns out to increase linearly with the residual resistivity, p(d, 0), of the samples. Moreover Ap(d, T ) is found to be substantially smaller after the sample surfaces had been roughened by chemical etching. These results are in quantitative agreement with recent size-effect calculations of Sambles et a1 at temperatures where A p ( c 0 , T ) < p ( m , 0). The conclusion to be drawn is that in the low-temperature limit, in conjunction with the theoretical predictions, surface scattering simply enhances the temperature dependence of the bulk resistivity by a constant factor.

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