We demonstrated the in situ one-step fabrication of suspended single wall carbon nanotube transistors with Fe/Al bi- layered film electrodes for practical integrated quantum phase devices. At 300 K, the devices show field effect transistor operation with an excellent subthreshold swing of S ~ 90 mV/decade for a long channel of 3 mum. In the low temperature regime, we observed four clear peaks corresponding to the four-fold degeneracy of the quantum energy levels at 3.7 K. These four clear peaks indicated that both of the contacts between the SWNT and Fe/Al are highly transparent and that a high-quality SWNT bridge is formed. The dl/dV characteristics under an applied external magnetic field indicate that the modulation of the bandgap of the nanotube with the oscillation of the conductance can be achieved by varying the magnetic field, due to the quantum interference of the electrons. In summary, the simple one-step grown SWNT junction between Fe electrodes can be utilized as a promising element for integrated quantum electronic devices.