▎ 摘　　要

Ever since the discovery of graphene(1), valley symmetry and its control(2,3) in the material have been a focus of continued studies in relation to valleytronics(4,5). Carrier-guiding quasi-one-dimensional (1D) graphene nanoribbons (GNRs)(6-12) with quantized energy subbands preserving the intrinsic Dirac nature have provided an ideal system to that end. Here, by guiding carriers through dual-gate operation in high-mobility monolayer graphene, we report the realization of quantized conductance in steps of 4e(2)/h in zero magnetic field, which arises from the full symmetry conservation of quasi-1D ballistic GNRs with effective zigzag-edge conduction. A tight-binding model calculation confirms conductance quantization corresponding to zigzag-edge conduction even for arbitrary GNR orientation. Valley-symmetry conservation is further confirmed by intrinsic conductance interference with a preserved Berry phase of pi in a graphene-based Aharonov-Bohm(AB) ring preparedby similar dualgating. This top-down approach for gate-defined carrier guiding in ballistic graphene is of particular relevance in the efforts towards efficient and promising valleytronic applications.