  In single electron tunneling through clean, suspended carbon nanotube devices
at low temperature, distinct switching phenomena have regularly been observed.
These can be explained via strong interaction of single electron tunneling and
vibrational motion of the nanotube. We present measurements on a highly stable
nanotube device, subsequently recorded in the vacuum chamber of a dilution
refrigerator and immersed in the 3He/4He mixture of a second dilution
refrigerator. The switching phenomena are absent when the sample is kept in the
viscous liquid, additionally supporting the interpretation of dc-driven
vibration. Transport measurements in liquid helium can thus be used for finite
bias spectroscopy where otherwise the mechanical effects would dominate the
current.
