  The paper reports on the improved Mainz experiment on tritum $\beta$
spectroscopy which yields a 10 times' higher signal to background ratio than
before. The main experimental effects and systematic uncertainties have been
investigated in side experiments and possible error sources have been
eliminated. Extensive data taking took place in the years 1997 to 2001. A
residual analysis of the data sets yields for the square of the electron
antineutrino mass the final result of $m^2(\nu_e)=(-0.6 \pm 2.2_{\rm{stat}} \pm
2.1_{\rm{syst}})$ eV$^2$/c$^4$. We derive an upper limit of $m(\nu_e)\leq 2.3$
eV/c$^2$ at 95% confidence level for the mass itself.

High Energy Physics - Experiment