@Book{ref,
  author = {{u c}},
  title = {Title}
}

documentclass{article}

usepackage[backend=biber, style=alphabetic]{biblatex}
usepackage[utf8]{inputenc}

bibliography{references}
begin{document}

cite{ref}

printbibliography
end{document}

! Package inputenc Error: Unicode char ̆ (U+306)
(inputenc)                not set up for use with LaTeX.

documentclass{article}

usepackage[backend=biber, style=alphabetic, texencoding=ascii]{biblatex}
usepackage[utf8]{inputenc}


usepackage{filecontents}
begin{filecontents}{jobname.bib}
@book{ref,
  author = {{u{c}}, breve},
  title = {Title}
}
end{filecontents}

addbibresource{jobname.bib}

begin{document}
cite{ref}

printbibliography
end{document}