documentclass[12pt]{article}

usepackage[T1]{fontenc}
usepackage[utf8]{inputenc}
usepackage[
    a6paper,
    showframe,
]{geometry}

newenvironment{myquote}[1]{%
  par
  begin{minipage}{dimexprtextwidth-2parindentrelax}%
    defmyquoteauthorname{#1}%
  }{%
    parvspace{1ex}
    noindent
    hspace*{0.25textwidth}%
      rule{0.5textwidth}{.4pt}
    parvspace{1ex}
    centering
    textsc{myquoteauthorname}par
  end{minipage}
  par}

begin{document}

begin{myquote}{Robert Kleinhenz}
    When asked what it was like to set about proving something, the mathematician
  likened proving a theorem to seeing the peak of a mountain and trying to climb
  to the top. One establishes a base camp and begins scaling the mountain's sheer
  face, encountering obstacles at every turn, often retracing one's steps and
  struggling every foot of the journey. Finally when the top is reached, one
  stands examining the peak, taking in the view of the surrounding countryside
  and then noting the automobile road up the other side!
end{myquote}

end{document}

documentclass[12pt, twocolumn]{article}

usepackage[T1]{fontenc}
usepackage[utf8]{inputenc}
usepackage[
    a4paper,
    showframe,
]{geometry}

newenvironment{myquotecolumn}[1]{%
  par
  begin{minipage}{dimexprcolumnwidth -2parindentrelax}%
    defmyquoteauthorname{#1}%
  }{%
    parvspace{1ex}
    noindent
    hspace*{0.25columnwidth }%
      rule{0.5columnwidth }{.4pt}
    parvspace{1ex}
    centering
    textsc{myquoteauthorname}par
  end{minipage}
  par}

begin{document}

begin{myquotecolumn}{Robert Kleinhenz}
    When asked what it was like to set about proving something, the mathematician
  likened proving a theorem to seeing the peak of a mountain and trying to climb
  to the top. One establishes a base camp and begins scaling the mountain's sheer
  face, encountering obstacles at every turn, often retracing one's steps and
  struggling every foot of the journey. Finally when the top is reached, one
  stands examining the peak, taking in the view of the surrounding countryside
  and then noting the automobile road up the other side!
end{myquotecolumn}

end{document}