Graphene nanoribbons have attracted attention because of their novel electronic and spin transport properties(1-6), and also because nanoribbons less than 10 nm wide have a bandgap that can be used to make field-effect transistors(1-3). However, producing nanoribbons of very high quality, or in high volumes, remains a challenge(14-18). Here, we show that pristine few-layer nanoribbons can be produced by unzipping mildly gas-phase oxidized multiwalled carbon nanotubes using mechanical sonication in an organic solvent. The nanoribbons are of very high quality, with smooth edges (as seen by high-resolution transmission electron microscopy), low ratios of disorder to graphitic Raman bands, and the highest electrical conductance and mobility reported so far (up to 5e(2)/h and 1,500 cm(2) V-1 s(-1) for ribbons 10-20 nm in width). Furthermore, at low temperatures, the nanoribbons show phase-coherent transport and Fabry-Perot interference, suggesting minimal defects and edge roughness. The yield of nanoribbons is similar to 2% of the starting raw nanotube soot material, significantly higher than previous methods capable of producing high-quality narrow nanoribbons(1). The relatively high-yield synthesis of pristine graphene nanoribbons will make these materials easily accessible for a wide range of fundamental and practical applications.