Our results prove the local origin of magnetoresistance in electrochemically deposited Ni nanocontacts. Experiments have been done using a complex setup for both in situ growth and ballistic magnetoresistance ͑BMR͒ measurements. Nanocontacts have been grown between two macroscopic Ni wires. In situ experiments with variation of the nanocontact diameter from 3 to 20 nm have been done using the same pair of wires. BMR values from 0.5% to 100% have been observed but no correlation of BMR value with the sample resistance, i.e., with the nanocontact cross section, has been found. These results show that the BMR in the nanometric size contact is determined by local geometrical and magnetic structures near the nanocontact rather than by the contact cross section itself. The hypothesis of existence of the intrinsic nonmagnetic dead layer in the ferromagnetic nanocontact is proposed to account for the BMR properties of the nanometric size contacts. Additionally, we report a BMR value of 200% in a Ni nanocontact ͑5 nm diameter͒ electrochemically grown between two nonmagnetic macroscopic gold wires. An external magnetic field has been used during the electrochemical deposition to fix the easy magnetic axis of the deposited Ni layer.Ballistic magnetoresistance ͑BMR͒ in ferromagnetic atomic size ͑less than 1 nm diameter͒ nanocontacts were first reported in 1999. 1 The physical origin of the large magnetoresistance ͑MR͒ values ͑up to 300% at room temperatures͒ is a modification of the spin-dependent transparency of the nanocontact by the external magnetic field that changes the orientation of magnetization in an area of a few nanometers near the contact. 2 The BMR is a local effect, i.e., local magnetization near the contact plays the dominant role. Atomic size contacts are stable for a few minutes only, which impairs technological applications. Subsequent investigations have shown very large BMR ͑up to 700%͒ at room temperatures for nanometric size ͑1-100 nm diameter͒ contacts. 3,4 These nanocontacts were electrochemically grown between two ferromagnetic wires and they are stable for days. Stable BMR structures can successfully compete with giant MR 5 and tunnel MR 6 structures for applications as local magnetic sensors or as reading magnetic heads. An important question, which is very significant for practical applications, is the role of the bulk ferromagnetic electrodes. The use of large ferromagnetic electrodes ͑5 mm long and 0.

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