Magnetoconductance of a Single-Electron Transistor in the Kondo Regime*

We have measured the zero-bias conductance of a Single-Electron Transistor (SET) in the Kondo regime as a function of temperature, $T,$ and magnetic field, $B,$ oriented parallel to the plane of the device. Our SETs are fabricated on a GaAs/AlGaAs heterostructure with electron sheet density $4.8\times 10^{11}$ cm$^{-2}$ and mobility $5\times 10^5$ cm$^2$V$^{-1}$s$^{-1}$. Scaled plots of both the $T$ and $B$-dependent data show universal behavior. At moderate and high $B$, the magnetoconductance data show good agreement with renormalization group calculations in the spin-1/2 Kondo regime. At very low $B$, we observe a non-monotonic behavior: as $B$ increases, the conductance initially increases and only starts to decrease at a finite $B$. A possible explanation of this effect due to the presence of multiple orbital dot levels with similar energies will be discussed.