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Variable range hopping (VRH) conduction in the insulating three-dimensional n-CdSe samples has been studied over the entire temperature range from 0.03 to 1 K. In the absence of a magnetic field, the low temperature conductivity of the three samples (A, B and C) obeys the Mott VRH conduction with an appropriate temperature dependence in the prefactor ( D 0 exp OE .T 0 =T / p with p 0:25/. This behavior can be explained by a VRH model where the transport occurs by hopping between localized states in the vicinity of the Fermi level, E F , without creation of the Coulomb gap (CG). On the contrary, no Efros-Shklovskii VRH is observed, suggesting that the density is constant in the vicinity of the E F .

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In this paper, positive magnetoresistance of a dilute two-dimensional electron gas in GaAs is studied in a parallel magnetic field B. It is found that the normalized resistivity curves, ρ(B)/ρ(0), merge together when we scale the field according to B/B χ , where B χ is assumed to be the field in which full spin polarization of electrons is reached. It is also shown that the crossing field, B cross , determined by the crossover of the B 2 dependence of the resistivity, becomes lower than B χ with increasing electron density, which cannot be explained in terms of filling of the upper electron subbands in the fully spin-polarized regime. The spin susceptibility, χ, is assessed by extracting the product g * m * , where g * and m * are the effective Landé factor and electron mass, respectively. The behavior of χ with increasing electron density, however, deserves further theoretical and experimental study.

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