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In two dimensions the frequency of magnetoconductivity oscillations measures the sum of Fermi-surface areas when the Landau levels are sharp. This explains why (100) Si inversion layers with two occupied subbands show only one frequency. It also explains why the frequency observed on (111) surfaces simulates a valley degeneracy of 2, even though six valleys are occupied.
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Vinter et al. (1980) studied this question.
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