In this paper, we discuss the transport mechanism in nitrogen-doped ultrananocrystalline (N-UNCD) and B-doped nanocrystalline (B-NCD) diamond thin films, which have recently attracted significant attention due to possible applications in electronics and bioelectronics. We present clear evidence that the transport in UNCD films at LHeT has low-dimensional quantum character and can be explained by a weak localisation (WL) model. Our model explains the negative magnetoresistance, observed in these films for the first time, and confirms the WL phenomena. For comparison, we have prepared thin B-NCD films, doped by B using TMB. Films with thickness of about 150 nm deposited on glass wafers are fully transparent and highly conductive. B-concentrations close to the Metal Insulating Transition (MIT) are confirmed by Raman measurements. We discuss the positive magnetoresistance data observed also for the first time in B-NCD films and compare the transport mechanism with UNCD films.