Yes, you increase the RPM slip of a "boxer" type fan by installing
a capacitor in series with the fan's power supply.  The air flow of
small 3.5 inch fans can be reduced by about 50% by using a 1 to 4
uF capacitor.  Use a good grade nonpolarized unit with working
voltage rating around 250 volts.  Note that some impriical study is
usually required to experimentally determine the best size
capacitor for a given application.

For DC powered applications, try the Radio Shack 12 volt box fan.
It can run and start reliably from as low as about 4.5 VDC.  It is
exceptionally quiet, but at admittedly low flow.  I wish I knew who
made the fans for Radio Shack.

