  Magnetoresistance in two-dimensional array of Ge/Si quantum dots was studied
in a wide range of zero-magnetic field conductances, where the transport regime
changes from hopping to diffusive one. The behavior of magnetoresistance is
found to be similar for all samples - it is negative in weak fields and becomes
positive with increase of magnetic field. The result apparently contradicts to
existing theories. To explain experimental data we suggest that clusters of
overlapping quantum dots are formed. These clusters are assumed to have
metal-like conductance, the charge transfer taking place via hopping between
the clusters. Relatively strong magnetic field shrinks electron wave functions
decreasing inter-cluster hopping and, therefore, leading to a positive
magnetoresistance. Weak magnetic field acts on "metallic" clusters destroying
interference of electron wave function corresponding to different paths (weak
localization) inside clusters. The interference may be restricted either by
inelastic processes, or by the cluster size. Taking into account WL inside
clusters and hopping between them within the effective medium approximation we
extract effective parameters characterizing charge (magneto) transport.
