  Metrological investigations of the quantum Hall effect (QHE) completed by
transport measurements at low magnetic field are carried out in
a-few-$\mu\mathrm{m}$-wide Hall bars made of monolayer (ML) or bilayer (BL)
exfoliated graphene transferred on $\textrm{Si/SiO}_{2}$ substrate. From the
charge carrier density dependence of the conductivity and from the measurement
of the quantum corrections at low magnetic field, we deduce that transport
properties in these devices are mainly governed by the Coulomb interaction of
carriers with a large concentration of charged impurities. In the QHE regime,
at high magnetic field and low temperature ($T<1.3 \textrm{K}$), the Hall
resistance is measured by comparison with a GaAs based quantum resistance
standard using a cryogenic current comparator. In the low dissipation limit, it
is found quantized within 5 parts in $10^{7}$ (one standard deviation, $1
\sigma$) at the expected rational fractions of the von Klitzing constant,
respectively $R_{\mathrm{K}}/2$ and $R_{\mathrm{K}}/4$ in the ML and BL
devices. These results constitute the most accurate QHE quantization tests to
date in monolayer and bilayer exfoliated graphene. It turns out that a main
limitation to the quantization accuracy, which is found well above the
$10^{-9}$ accuracy usually achieved in GaAs, is the low value of the QHE
breakdown current being no more than $1 \mu\mathrm{A}$. The current dependence
of the longitudinal conductivity investigated in the BL Hall bar shows that
dissipation occurs through quasi-elastic inter-Landau level scattering,
assisted by large local electric fields. We propose that charged impurities are
responsible for an enhancement of such inter-Landau level transition rate and
cause small breakdown currents.
