Quantum resistance metrology in graphene
  We have performed a metrological characterization of the quantum Hall resistance in a 1 $\mu$m wide graphene Hall-bar. The longitudinal resistivity in the center of the $\nu=\pm 2$ quantum Hall plateaus vanishes within the measurement noise of 20 m$\Omega$ upto 2 $\mu$A. Our results show that the quantization of these plateaus is within the experimental uncertainty (15 ppm for 1.5$ \mu$A current) equal to that in conventional semiconductors. The principal limitation of the present experiments are the relatively high contact resistances in the quantum Hall regime, leading to a significantly increased noise across the voltage contacts and a heating of the sample when a high current is applied. 
A. J. M. Giesbers, G. Rietveld, E. Houtzager, U. Zeitler, R. Yang, K.   S. Novoselov, A. K. Geim, and J. C. Maan