We study the crossover from metallic to insulating behaviour of the 2DEG in a vicinal Si MOSFET and 2DHG in a GaAs/GaAlAs heterostructure. It is shown that the crossover complies with a conventional Fermi-liquid approach. In the electron gas, it is caused by an impurity band at the Si/Si oxide interface. In the hole gas, the negative magnetoresistance near the crossover is well described by weak localisation, in spite of large values of interaction parameter rs ~ 30. We also study ρ(T) and parallel magnetoresistance in the metallic regime. It is shown that this regime is caused by phonon-scattering and holehole interaction correction in a disordered Fermi liquid.