Studies of Nanoconstrictions Nanowires and Fe3 O4 Thin Films : Electrical Conduction and Magnetic Properties. Fabrication by Focused Electron Ion Beam This work constitutes a detailed study of electrical and magnetic properties in nanometric materials with a range of scales: atomic sized nanoconstrictions micro and nanowires and thin films. Firstly a novel method of fabricating atomic sized constrictions in metals is presented; it relies on measuring the conduction of the device while a focused ion beam etching process is in progress. This work constitutes a detailed study of electrical and magnetic properties in nanometric materials with a range of scales: atomic sized nanoconstrictions micro and nanowires and thin films. Firstly a novel method of fabricating atomic sized constrictions in metals is presented it relies on measuring the conduction of the device while a focused ion beam etching process is in progress. Secondly it describes wires created by a very promising nanolithography technique: Focused electron ion beam induced deposition. Three different gas precursors were used: (CH3)3Pt(CpCH3) W(CO)6 and Co2(CO)8. The thesis reports the results obtained for various physical phenomena: the metal insulator transition superconducting and magnetic properties respectively. Finally the detailed magnetotransport properties in epitaxial Fe3O4 thin films grown on MgO (001) are presented. Overall the new approaches developed in this thesis have great potential for supporting novel technologies.