  Graphene's isolation launched explorations of fundamental relativistic
physics originating from the planar honeycomb lattice arrangement of the carbon
atoms, and of potential technological applications in nanoscale electronics.
Bottom-up fabricated atomically-precise segmented graphene nanoribbons, SGNRs,
open avenues for studies of electrical transport, coherence, and interference
effects in metallic, semiconducting, and mixed GNRs, with different edge
terminations. Conceptual and practical understanding of electric transport
through SGNRs is gained through nonequilibrium Green's function (NEGF)
conductance calculations and a Dirac continuum model that absorbs the
valence-to-conductance energy gaps as position-dependent masses, including
topological-in-origin mass-barriers at the contacts between segments. The
continuum model reproduces the NEGF results, including optical Dirac
Fabry-Perot (FP) equidistant oscillations for massless relativistic carriers in
metallic armchair SGNRs, and an unequally-spaced FP pattern for mixed
armchair-zigzag SGNRs where carriers transit from a relativistic (armchair) to
a nonrelativistic (zigzag) regime. This provides a unifying framework for
analysis of coherent transport phenomena and interpretation of forthcoming
experiments in SGNRs.
