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We propose an analytical description of the role of local depolarization fields in the terahertz conductivity of nanostructured samples and demonstrate this approach in a sample composed of silicon nanocrystals. This helps to uncover the nature of charge carrier transport at nanoscale. Time-resolved terahertz conductivity is investigated in an ensemble of silicon nanocrystals fabricated by electrochemical etching of silicon wafer followed by an H _2 O _2 oxidizing treatment. The post-etching treatment leads to a decrease in the average nanocrystalline Si core size which enhances luminescence in the visible range. We show that the dominating microscopic photoconductive response of photocarriers is essentially Drude-like owing to the presence of a very small amount of large nanocrystals; the macroscopic character of the response is, however, deeply modified by the depolarization fields. Smaller nanocrystals appreciably contribute to the terahertz conductivity only at high photoexcitation densities where the screening due to depolarization fields suppresses the response of the large particles.