How can I type "lambda-bar"?

As far as I know (confirmed by the Comprehensive list of LaTeX symbols), the only package that provides \lambdabar is txfonts (and the derived newtxmath). Of course changing all document fonts to get that symbol is out of the question (and the lambda is quite different from other fonts, so it's also impossible to import only it).

One can build the symbol similarly to \hbar:

\documentclass{article}
\newcommand{\lambdabar}{{\mkern0.75mu\mathchar '26\mkern -9.75mu\lambda}}
\begin{document}
$|\lambdabar|$

$|\lambda|$
\end{document}

This exploits the fact that the bar character used is exactly 9mu wide. In the standard definition of \hbar there is just \mkern-9mu, but we need to push the bar slightly to the right. (The second line is just to compare the results.)

---

A different implementation, where the bar is lower.

\documentclass{article}

\makeatletter
\newcommand{\lambdabar}{{\mathchoice
  {\smash@bar\textfont\displaystyle{0.25}{1.2}\lambda}
  {\smash@bar\textfont\textstyle{0.25}{1.2}\lambda}
  {\smash@bar\scriptfont\scriptstyle{0.25}{1.2}\lambda}
  {\smash@bar\scriptscriptfont\scriptscriptstyle{0.25}{1.2}\lambda}
}}
\newcommand{\smash@bar}[4]{%
  \smash{\rlap{\raisebox{-#3\fontdimen5#10}{$\m@th#2\mkern#4mu\mathchar'26$}}}%
}
\makeatother


\begin{document}

$|\lambdabar|_{\lambdabar_\lambdabar}$

$|\lambda|_{\lambda_\lambda}$
\end{document}

I've left the four parameters to \smash@bar so that one can fine tune them for different fonts. The variable parameters are the third (amount of shifting down as a fraction of 1ex in the correct font size) and the fourth (amount of shifting right of the bar, in mu units).

It's provided by revsymb in revtex4. So you could either use the revtex4 class or just \usepackage{revsymb} and then write $\lambdabar$. Here's what you get:

This symbol has a unicode with the number U+019B. You may choose some font which supports this symbol and use it as shown below:

% arara: lualatex 

\documentclass{article}
\usepackage{fontspec}
\usepackage{mathtools}
\newcommand{\lambdabar}{{\setmainfont{Linux Libertine O}\text{\symbol{"019B}}}}

\begin{document}
%In text \lambdabar, in inline math $\lambdabar$ or in display:
\[|\lambdabar|_{\lambdabar_\lambdabar}\]
\end{document}