Electrical transport properties of thin boron doped nanocrystalline diamond films with thicknesses of $60–500\mathrm{nm}$ have been studied. The Raman spectra measured exhibit Fano resonances, characteristic for B concentrations close to the metal-to-insulator transition. Upon increasing the B concentration, the $s{p}^2$ carbon related Raman resonances vanish. In such boron-doped nanocrystalline diamond films, a positive magnetoresistance could be observed at liquid helium temperatures. The boron doped diamond films show conductivity similar to that of B-doped epitaxial diamond without any significant contribution of the grain boundary transport, leading to the superconductive transition in nanocrystalline diamond at $\sim 1.66\mathrm{K}$ .

In this letter we present results of magnetotransport measurements, carried out on heavily nitrogen-doped ultrananocrystalline diamond films, prepared by plasma-enhanced chemical vapor deposition. This material having at room temperature appreciably high electric conductivity $(\sim 4200\mathrm{S}{\mathrm{m}}^{-1})$ revealed surprisingly at Kelvin temperatures a giant negative magnetoresistance reaching up to 22% at $1.28\mathrm{K}$ and at $8\mathrm{T}$ . The analysis of experimental data has borne evidence of the fact that the transport in this subsystem has a character of low-dimensional disordered metal and it is controlled by quantum interference effects of electrons resulting in their weak localization.